Strict developmental control of antigen receptor gene assemble is essential to normal lymphopoiesis and the ability to mount an immune response to pathogenic insult. An understanding of the mechanism by which this control is imposed is prerequisite to the rational design of treatments for related immunologic defects. Prior studies show that recombination and expression of TCRBeta miniloci depends upon the presence of active enhancer. By altering transcription regulatory sequences with the TCRbeta minilocus, this model substrate will be further exploited to 1) determine whether transcription alone is sufficient to modulate rearrangement of a TCRbeta minilocus, 2) assess the individual contributions of RNA polymerase read through and transcription factor binding to accessibility, and 3) determine whether efficient rearrangement is associated with specific alterations in substrate chromatin. To model recombinational accessibility, consequences of alterations in the placement and composition of transcriptional control elements on the rearrangement of TCRbeta miniloci will be assessed following stable substrate introduction into a novel recombinase-inducible cell line, TCR- 19. Together, these studies will provide a definitive assessment of the roles played by transcriptional control elements in the regulation of antigen receptor gene assembly during lymphocyte development.